Tamera Eplin, PEAssistant Regional Engineer Mooresville Regional Office

Land Quality Section

Inundation movie clip

Inundation maps developed by identifying potential at risk structures and roads without a detailed engineering analysis and/or modeling.

Phot0-based maps are preferred.

SIMS are most applicable for:Small or medium sized dams with an easily-identified number of downstream structures, where local emergency management agree adequate evacuation procedures may be established.

A small or medium sized dam without funding for engineering studies. SIMS used as interim until funding can be arranged and the mapping updated.

More detailed modeling/analyses may be warranted for:

Large sized dams,Dams with large populations in the evacuation area,Dams with diversion structures, split flows, or potential for “domino-style” cascading failures of dams downstream.Flood control structures with large amounts of freeboard.

Dam

Dam

DisclaimersSIMS guidance is not meant to replace any state or federal regulatory requirement.

SIMS may not replace inundation mapping for assessment of downstream hazard potential.

SIMS may not be used for design or rehabilitation of dams.

EAP Development should involve local emergency management to establish notification and evacuation procedures.

North Carolina Simplified Inundation Maps for Emergency Action Plans

Please refer to December 2010 Guidance provided in your handouts for full recommendations.

Obtain an aerial view of the area downstream of the dam and identify potential at-risk structures.

Your handouts have suggested sources for free or low cost aerial views.

The following map was made using Google Earth.

•Obtain a topographic or contour map of the area downstream.

Assume the initial breach wave height just below the dam is about half the height of the dam.

For example, the initial breach wave height of a dam 20 feet tall is 10 feet.

Assume the breach wave height would be halved every 10 miles downstream.

Determining Breach Flood Wave Height

Distance Downstream of DamAssumed Breach Flood Wave Height, where H=height of dam

Just below the dam 0.5H

0.5 miles 0.488H

1 miles 0.475H

2 miles 0.45H

3 miles 0.425H

5 miles 0.375H

10 miles 0.25H

Distance to measure downstreamVaries based on surface area of the dam.

The distance may need to be extended, based on population density at the end of the recommended distance.

Surface Area of Dam Pond,At Normal Pool

Recommended Distance Downstream of Dam

Less than 25 acres 2 miles

25 to less than 100 acres 5 miles

100 acres or more 10 miles

Distance to measure downstream (continued)

Measure downstream until the breach wave is generally contained within the stream channel limits

And/or

To the point where no other downstream structures or roads will be impacted.

An Important Note

The SIMS map does not replace the list of downstream hazards in the Emergency Action Plan.

The SIMS map should be updated on a periodic basis.

Using the topographic or contour map, develop a scale (i.e. 1 cm = 200 ft)

select points downstream of the dam where contours cross the stream

Draw a line perpendicular to the stream at each point.

Step 6At each line, measure distance to the dam and determine the breach wave height from Table 1 in guidance document.

Use this height to count up from the stream bed on both sides and draw a point. For example, if the breach wave height is estimated to be 20 feet at a distance downstream of the dam and your map has 10-foot contour intervals, draw points approximately 2 contour intervals from the streambed.

Connect the points drawing two boundary lines from the dam down to the end of your inundation area.

If necessary, transfer data from the topographic map to the aerial photo and identify any potential at-risk structures.

List all Downstream HazardsIf practical, label hazards on SIMS map for easy reference during evacuation.

In closing…Developing inundation maps is an inexact science. SIMS do not replace engineering judgment.

SIMS produce conservative inundation limits for certain conditions.

Ultimately, the dam owner, the Dam Safety Program, and emergency management personnel must agree to an EAP, using the most representative inundation map available.

Please do not hesitate to contact the North Carolina Dam Safety Staff at (919) 733-4574.